Present day passenger vehicles are increasingly equipped with camera-based assistance systems, which capture the surrounding environment of the vehicle, and provide a variety of functions for improving driving safety and comfort. The functionality of these systems is based on the analysis of the recorded image data. Therefore, the quality of the system's predictions is directly related to the quality of the image data.
One factor that affects the image quality and which is difficult to control is the degree of contamination of the optical system of the camera. The cameras may be positioned at places with less risk of contamination, or the cameras may be cleaned by an electric wiper. Despite of these provisions, it is impossible to avoid a contamination of the optical system completely. Therefore, it has been proposed to detect dirt particles on a camera lens automatically in order to trigger an appropriate action. An example for such an automatic detection of lens contaminations is disclosed in the European patent application EP 2351351.